Ion source with side and end walls having independent potentials



May 6, 1969 w. BROWN 3,443,088

ION SOURCE WITH SIDE AND END WALLS HAVING INDEPENDENT POTENTIALS FiledMarch 16, 1966 Sheet l or 2 ID r r T r r r W; PUMP GAS i SOURCEAMPLIFIER 2 1 M I P INVENTOR.

HARMON W, BROWN BY ORNEY May 6, 1969 H. w. BROWN ION SOURCE WITH SLIDEAND END WALLS HAVING INDEPENDENT POTENTIALS Sheet Filed March 16, 1966 NA m 4 Q 6 Ta I 2 1J 3 u N 7 |l 4 1J y 2 00 E f "NT Wm H m mw E O L v nnlHlH/V MQMMM/AW HHM m mm V w w v m0 w 20 J 2 H \i v FEGB N2(MASS 2a)N(MASS I4) INVENTOR. W. BROWN RNEY United States Patent Office 3,443,088Patented May 6, 1969 US. Cl. 250-41.9 8 Claims ABSTRACT OF THEDISCLOSURE A cycloidal mass spectrometer is disclosed. The massspectrometer includes an improved ion source. The ion source includes asubstantially hollow cylindrical chamber having a cylindrical side walland a pair; of disc shaped end walls. An electron beam is passed throughthe center of the chamber along a path falling in a midplane parallel tothe plane of the end walls. The electron beam serves to ionize gasflowing into the ionizing chamber through one of the end walls toproduce an ion beam which exits through a beam defining slot in theother end wall. The cylindrical side wall is operated at a potentialintermediate the potentials applied to thev end walls such that the ionsare produced in a relatively strong accelerating field which rapidlyaccelerates the' ions out of the ionizing beam path. By causing theionizing beam to cross the ionizing chamber parallel to an eqpipotentialplane in the central region of the ionizing chamber, the ions areproduced in a region of uniform accelerating electric field and byelectrons of a well-defined potential. This facilitates obtaining highresolution in the output of the spectrometer and facilitatesdetermination of ionizing and dissociation potentials. Means are alsoprovided for varying the beam potential.

Heretofore ion sources used in mass spectrometer have had severalproblems. First, they have been characterized by relatively lowsensitivity as of 02x10 amps/torr for high resolution spectrometers,i.e., ion beam exit slit widths yielding mass resolution greater than1000. Typically this low sensitivity is caused by one or more factors.One factor lea-ding to low sensitivity is the production of ions in aregion of low accelerating electric field, i.e., less than 100 v./cm.such that the ions, once produced, are not quickly removed from thesource. A second characteristic of some of the prior ion sources hasbeen their poor ionization and dissociation potential resolution. Onefactor which causes this poor potential resolution is the production ofthe ions by electrons that must cross equipotentia'ls in the ionizingregion, whereby the electrons ionize or dissociate gaseous material atdifferent potentials along the directon of their ionizing trajectories.The result is that ionizing and dissociation potentials of the sourceare not well defined. Furthermore, many prior art ions sources have beenrelatively open structures permitting leakage of gas therefrom withoutbeing ionized. As a result the mass spectrometer apparatus isunnecessarily contaminated by the wasted material and furthermore moresample material is required than would otherwise be used.

In the present invention an improved ion source is provided whichionizes and/or dissociates the gaseous material to be analyzed in arelatively gas tight chamber and within a region thereof of relativelyintense uniform electric field with the ionizing and/or dissociatingelectrons entering the ion production region in the plane of theequipotentials. In this manner ions are efficiently produced at welldefined ionizing or dissociation potentials and the ions, once produced,are quickly removed through the ion beam exit slit, whereby an order ofmagnitude increase in sensitivity is obtained for a given high massresolution.

The principal object of the present invention is the provision of animproved ion source for use in cycloidal mass spectrometers.

One feature of the present invention is the provision of a substantiallyclosed ionizing chamber in the ion source having a minimum of gasleakage therefrom, other than through the ion beam exit slit, wherebyefficient ionization of the gas introduced thereto is obtained.

\Another feature of the present invention is the provision of an ionsource wherein the gas is ionized, within an ionizing chamber, in aregion of relatively intense uniform electric field having an intensitygreater than v./cm., whereby ions produced are rapidly removed from theionizing region of the source through the beam exit slit.

Another feature of the present invention is the same as any one or moreof the preceding wherein the ions are produced in a region of electricand/or structural symmetry of the ionizing chamber to assure uniformityof the electric field in which the ion beam emanates.

Another feature of the present invention is thesarne as any one or moreof the preceding wherein the ions are produced in a region of uniformelectric field by an elec tron beam which passes through the ionizingregion substantially parallel to and in the plane of the electricequipo-tentials within the ion beam source region of the ionizingchamber, whereby the ionizing and/or dissociation potentials of the ionsource are well defined.

Another feature of the present invention is the same as the precedingwherein the ion source includes an ionizing chamber formed by threeelectrodes, a pair of spaced separate end wall electrodes separated byan intervening ring electrode portion operating at a potentialintermediate the potentials applied to the end wall electrodes, andwherein the ionizing region is located centrally of the ring electrode.

Other features and advantages of the present invention will becomeapparent upon a persual of the following specificaion taken inconnection with the accompanying drawings wherein:

FIG. 1 is schematic drawing of a cycloidal mass spectrometer systememploying features of the present invention,

FIG. 2 is a circuit diagram of the network for applying operatingpotentials to the electric field ion analyzer electrode array of FIG.-1,

FIG. 3 is an enlarged sectional view, partly schematic, of the ionsource structure of FIG. 1 taken along line 3-3 in the direction of thearrows,

FIG. 4 is a view of the structure of FIG. 3 taken along line 44 in thedirection of the arrows, and

FIG. 5 is a plot of detected ion current for N and N+ versus electronvolts of the ionizing electron beam for the ion source of FIG. 3.

Referring now to FIG. 1 there is shown a cycloidal mass spectrometersystem. More particularly, an array of generally rectangular shaped ringelectrodes 1 are ins-ulatively supported within a thin rectangularvacuum envelope 2, only partially shown, from a heavy rectangularflange, not shown, which closes off one end of the vacuum envelope.

The separate rings 1 of the electrode array are operated at slightlydifferent electric potentials derived from a voltage source 3 via leads4 connected at nodes 5 of a voltage divider network 60. The differentpotentials applied to the different rings 1 establishes a region ofuniform electric field E in the hollow interior of the ring electrodearray. The electric field E is directed parallel to the line ofdevelopment of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field Hdirected at right angles to the direction of the electric field E. Thefield H is conveniently produced by an electromagnet 7 with the vacuumenvelope 2 being disposed in the narrow gap defined between a pair ofpole pieces 8 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably lowpressure as of l0 torrs. Gas to be analyzed by the analyzer section,including the array of electrodes 1, is introduced from a source 9 intothe analyzer section through the vacuum envelope 2 via an inlet tubing11 as of stainless steel. The inlet tubing 11 feeds gas at a desiredrate into an ion source 12. The ion source ionizes the gas and projectsit through a slot into the crossed magnetic field H and electric field Eof the analyzer.

Under the influence of the crossed electric and magnetic fields the ionsare caused to execute cycloidal trajectories. However, only ions of acertain mass number, for a given intensity of E and H, will be focusedat a detector slot 13 a certain focal distance from the source and atthe same electric potential. An ion detector 14 is positioned behind theslot 13 to produce an output corresponding to the number of ions underanalysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signaland feeds it to the Y axis of an X-Y recorder 16 wherein it is recordedas a function of a scan of the magnetic field intensity H produced by ascan generator 17. The output of the recorder 16 is a mass spectrum ofthe sample under analysis.

Referring now to FIGS. 3 and 4, the ion source -12 includes a metallicionizing chamber 21 as of stainless steel which may be rhodium plated toreduce corrosion and contamination and within which gas to be analyzedis ionized and formed into a beam 22. The ionizing chamber 21 issegmented and separated by thin insulating 'sheets 23 as of 0.005" thickmica to provide three separate electrodes 24, 25 and 26. The centerelectrode 25 includes a hollow cylindrical bore as of 0.250" in diameterand 0.116 in axial length defining the central portion of the ionizingchamber 21. The ends of the ionizing chamber 21 are closed off bytransverse walls 27 and 28 forming portions of electrodes 24 and 26,respectively. End wall 27 is centrally apertured to form a gas inletpassageway 29 in gas communication with an insulating section of the gasinlet pipe 11 for introducing gas, to be analyzed, into the ion source12. The opposite end wall 28 includes an ion beam exit slit 31 formed bya pair of slightly spaced apart knife edge plates 32 as of stainlesssteel sealed over a cylindrical 'bore 34 centrally located of the endwall 28. Bore 34 is, for example, 0.200" in diameter and the beam exitslit 31 is approximately 0.001 to 0.0004 in width as defined by thespacing between the plates 32. The elongated axis of the ion beam exitslit 31 is parallel to the direction of the magnetic field H whichthreads through the ion source 12 and ion analyzer rings 1. The gasinlet end wall 27 is counter bored at 35 to provide mechanical symmetrywith the bore 34 in the ion beam exit wall 28.

A pair of cylindrical electron beam passageways 36, axially aligned withthe direction of the magnetic field H, and as of, for example, 0.040" indiameter, pass through the inner wall of the center electrode 25. Thepassageways 36 define an electron beam path 37 therebetween coincidingwith and lying within the transverse structural plane of symmetry of theionizing chamber 21. A filamentary thermionic emitter 38 is exiallyaligned with the beam passageways 36 for projecting a beam of electronsacross the ionizing chamber over the beam path 37. The emitter 38 isheated by a current drawn from a battery 39. The central electrode 25serves as the anode for the emitter 38 and the anode potential for theemitter 38 is supplied from a variable voltage power supply 41 connectedbetween the filament 38 and its anode 25. The electron beam 37 serves toionize and/or to dissociate gas particles within the electron beam path37 inside the ionizing chamber 21 and is collected by a metalliccollector electrode 40 operating at anode potential and covering overthe beam exit hole 36.

Electrode 24 serves as the repeller electrode for the ion source 12 andis supplied with its independent operating electrical potential as of160-200 volts from a variable voltage source 42. Electrode 26 serves asthe beam exit electrode and is preferably operated at ground potential.-

The intermediate electrode 25 serves to produce a region of uniformintense electric field E as of more than volts/cm. over the centralionizing region 43 of the beam path 37 defined by the shaded region ofthe drawing, The central electrode 25 is preferably operated at apotential midway between the operating potentials applied to electrodes24 and 26. The operating potential for the central electrode 25 isderived from a centertap 44 of a voltage dividing network 45 formed byresistors 46 and 47 as of 10 K9 each connected across the voltage supply42.

In operation, gas to be analyzed by the cycloidal mass spectrometer isintroduced into the ion source 12 via gas inlet pipe 11, 11 and inletpassageway 29. The gas is ionized by the electron beam in the beam path37. Under the influence of the uniform electric field E, produced by thesystem of electrodes 24, 25 and 26, the ions within the central beampath region 43 are rapidly swept through the ion beam exit slit 31 toform a well defined ribbonshaped ion beam 22 emerging from the exit slit31. The central ring shaped electrode 25, operated at a potential midwaybetween the repeller and exit electrode potentials and placed in aposition of structural symmetry, allows the ionizing region 43 to beplaced in a position of optimum electric field uniformity. By making theinside diameter of ring 25 larger than the axial length, the intensityof uniform electric field is made relatively large as of greater than150 volts/ cm. averaged over the ionizing region 43. Thus, ions producedare rapidly withdrawn through the exit slit 31. As a result, the ionsource 12 yielded a sensitivity of 2 10- amps/torr with exit anddetector slits of the aforementioned dimensions giving a detected massresolution greater than 1000 between half amplitude points on thedetected mass peak.

Passing the ionizing electron beam path through the chamber 21 in aplane of electrical symmetry with the electrons directed parallel to theequipotentials of the uniform electric field E yields substantiallyimproved definition of the ionizing and dissociation potentials of theion source 12. For example, nitrogen gas introduced into the ionizingchamber 21 may undergo either one of the following reactions:

The first reaction (1) results in only ionizing the nitrogen gas toproduce N ions with mass number 28. While monitoring this mass number onthe mass spectrometer and decreasing the ionizing electron beam anodevoltage, a plot of ion current versus ionizing electron volts isobtained as shown in FIG. 5. The point where the mass 28 ion goes tozero represents the ionizing potential in electron volts for thenitrogen gas under analysis. This is of importance to chemists and it isdesired that this point be well defined. The ion source of the presentinvention permits good resolution of ionizing potential.

The second reaction (2) represents dissociation of the nitrogen gasmolecule and the potential at which this occurs is of interest tochemists and, therefore, should be well defined. This potential ismeasured in the same way as the ionizing potential, only mass 14 ismonitored instead of mass 28. The ion source 12 provides a well definedvalue for this potential as well.

Lastly, the ion source should not be wasteful of gas to be analyzed asunnecessary leaks in the ionizing chamber produce wasting of the sampleand contamination of the spectrometer. In the ion source 12 of thepresent invention, with a beam exit slit 31 of the aforementioneddimensions, the chamber 21 was free of unnecessary leaks to the extentthat the total leak rate taken through the source 12 from the inlet 29for all openings, including the beam exit slit, was less than 2liters/second for nitrogen gas.

The ion source 12 has been described as it would be used to produce apositive ion beam. However, the source is equally useful for producingnegative ion beams by merely reversing the terminals of the voltagesupply 42. The negative ion beam would be analyzed by reversing thedirection of the magnetic field H, and the direction of electric fieldE.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is: s

1. An ion source apparatus for providing a beam of ions for a cycloidalmass spectrometer including, means defining an ionizing chamber having agas input passageway leading thereto for introduction of gaseousmaterial to be ionized and subsequently mass analyzed, said chambermeans having an ion beam exit slit in gas communication therewiththrough which the ion beam emerges from the ion source for mass analysisby the mass spectrometer, said chamber means having a pair ofelectrically conductive spaced end walls separated by an interveningregion bounded by a surrounding electrically conductive side wallportion, said ion beam exit slit being disposed in one of said endwalls, means for insulating said end and side walls each from the otherto hold independent operating potentials, said chamber means having apair of axially aligned beam passageways passing through opposite wallportions thereof and defining a beam path therebetween for passage of anelectron beam through said chamber means along the beam path forionizing gas within said chamber means, said beam passageways beinglocated in said intervening surrounding side wall substantially midwayof the length of its intervening portion to direct the beam path acrosssaid chamber approximately parallel to equipotential planes in thecentral region of said chamber, whereby ions are produced in a region ofuniform electric field and by electrons of equal potential.

2. The apparatus according to claim 1 including means for applyingoperating potentials to said end and intervening side walls, saidpotential applying means applying a potential to said side wall portionwhich is intermediate the operating potentials applied to said endwalls.

3. The apparatus according to claim 1 wherein said beam passagewaysdefine a beam path therebetween which lies within a plane of structuralsymmetry inside of said chamber.

4. The apparatus according to claim 2 wherein said electron beampassageways define a beam path therebetween which lies within a plane ofelectrical symmetry inside of said chamber.

5. The apparatus according to claim 1 wherein said intervening side wallportion has a characteristic minimum inside transverse dimension whichis within i25% of being twice the axial extent of said intervening sidewall portion, whereby a uniform central region of relatively intenseelectric field is produced centrally of said chamber coextensive with acentral portion of the electron beam path.

6. The apparatus according to claim 1 wherein said ionizing chamber issubstantially gas tight except for said gas inlet, ion beam exit slitand electron beam passageways and wherein the degree of gas tightness ofsaid ionizing chamber is defined by its leak rate taken from said gasinlet through all other leaks and passageways and is less than 2liters/second for N gas.

7. The apparatus according to claim 2 wherein the applied operatingpotentials are of such a magnitude combined with the dimensions of saidionizing chamber to produce a central ionizing region within saidchamber traversed by the electron beam wherein the applied potentialsproduce an average electric field intensity greater than volts/cm.

8. The apparatus according to claim 1 in combination wtih a cycloidalmass spectrometer for mass analyzing the ion beam exiting from said ionbeam exit slot of said ionizing chamber means.

References Cited UNITED STATES PATENTS 2,975,277 3/1961 Von Ardenne313231 2,977,470 3/ 1961 Robinson. 3,265,890 8/1966 Briggs 250419 RALPHG. NILSON, Primary Examiner. S. C. SHEAR, Assistant Examiner.

